Micromechanical magnetic field sensors are generally available. German. Patent No. DE 44 42 441 A1 and European Patent No. EP 1 052 519 B1, for example, describe magnetic field sensors of the so-called “fluxgate” type, which include a semiconductor substrate, an excitation element, two detection coils, and a magnetic core. The excitation element, in this context, includes in each case an excitation coil, which is operated using an alternating current having an excitation frequency, whereby in the magnetic core a magnetic flux according to the magnetic hysteresis (B-H curve) of the same frequency is generated. The magnetic core includes a ferromagnetic material, which is brought periodically to magnetic saturation by the excitation element. In this instance, magnetic saturation means that an additional external field does not generate any substantial increase in the magnetic flux in the magnetic core. The magnetic flux in the magnetic core is proportional to the product of magnetic permeability and magnetic field. The permeability is accordingly comparatively low in the saturation range, whereas it is comparatively high in the zero crossing of the magnetic hysteresis. Due to the non-linearity of the permeability, a present external magnetic field, that is to be measured, causes a distortion of the magnetic flux, which is detectable with the aid of the detection coils. For this purpose, the detection coils are positioned in a contrary sense about the magnetic core, so that the sum of the induction currents induced in the detection coils by the magnetic flux in the magnetic core is equal to zero in the absence of an external magnetic field that is to be measured. In the presence of an external magnetic field that is to be measured, because of the nonlinear permeability of the ferromagnetic magnetic core, each of the induction currents includes different harmonics of the excitation frequency, which are a function of the external magnetic field. The sum of the induction currents is therefore not equal to zero and is a measure for the strength of the external magnetic field that is to be measured.
Conventionally, the excitation coil includes a coil cross section which is aligned perpendicular to a main plane of extension of a substrate of the system, the coil cross section being penetrated centrically by the magnetic field. It is disadvantageous in this system that the flux-conducting structure is not able to be implemented in the system in a standard CMOS process. Based on the materials nickel and iron that are unusual in the CMOS process, there is increased danger of contamination, so that this step is not applicable in many CMOS lines. In addition, conventionally, an excitation coil is provided that has a coil cross section that is parallel to the main plane of extension. Because of the planar design of this excitation coil, the implementation of such a magnetic field sensor takes up comparatively much space and is thus costly.